1. FIELD OF THE INVENTION
This invention is directed to solvent compositions and to a two-step liquid-liquid process for the selective recovery of hydroxide ion from alkaline waste solutions, including solutions that are highly concentrated in salts. The process entails contacting an alkaline aqueous solution containing an alkali metal hydroxide with a solvent containing a fluorinated alcohol extractant dissolved in a water-immiscible organic diluent. Upon contact, alkali metal hydroxide equivalents are effectively extracted into the solvent phase. Alkali metal hydroxide is subsequently recovered from the solvent by contacting the metal-containing organic phase with an aqueous stripping solution. The stripping process also regenerates the solvent for use in subsequent extraction-stripping cycles. Extractant acidity can be fine-tuned by strategic substitution of electron donating and/or electron withdrawing groups in the backbone chain of alkyl and arylalkyl fluorinated alcohol extractants to accommodate a variety of aqueous pH conditions.
2. DESCRIPTION OF THE PRIOR ART
The ability to recover hydroxide from an aqueous mixture is a potentially useful technology for treatment of caustic industrial process streams. Caustic process streams are common in a multitude of industrial manufacturing and research facilities. In many cases, these waste streams are neutralized with acid prior to disposal in a landfill. In the nuclear industry, treatment is not quite as simple.
Much DOE research has been aimed at the removal of radioactive constituents of high-level alkaline tank waste. At Hanford, Washington and other sites, a need has arisen to remove bulk constituents that would otherwise have to be vitrified in the resultant low-level waste (LLW) stream. Such a need was recently identified on the world-wide web site of the DOE Office of Environmental Management Tanks Focus Area (Technology Needs/Opportunities No. RL-WT008). This need is easily understood. For example, if the estimated 6.8 .times.10.sup.7 kg of sodium inventory of Hanford tanks is vitrified in the low-level borosilicate glass, each glass canister having a mass of 1650 kg and containing 15% sodium, then one expects a production of 280,000 glass canisters. If these are produced at five hundred thousand dollars per canister, then the low-level vitrification alone will cost one hundred forty billion dollars. A technical approach to decreasing the cost of the LLW vitrification would be to cleanly separate the major sodium salts from the waste. Perhaps the most obvious case for recycle is sodium hydroxide, a chemical which is needed for tank sludge washing and waste retrieval. Without such recycle, the needed sodium hydroxide would have to be purchased and added to the waste stream, ultimately increasing the waste volume and worsening the overall problem. It is estimated that 32% of the sodium in the Hanford tanks could be recovered as sodium hydroxide, half of which is needed for recycling. It is important to point out that the sodium hydroxide treatment step typically would follow the fission product (i.e., Cs-137, Sr-90, Tc-99) separation steps. Thus, long-term radiolytic stability of the solvents used for hydroxide recovery is not expected to be a significant issue.
It is also anticipated that private industries not associated with radioactive waste issues would be interested in use of the present invention. Caustic process streams are common in many of industrial manufacturing and research facilities including paper manufacturing and aluminum production. In many cases, these waste streams are neutralized with acid prior to disposal in a landfill. The present invention potentially provides a cost effective strategy to treat alkaline waste streams and recover sodium hydroxide. The recovered product could then be recycled for use in commercial applications. Also, if recovery of sodium hydroxide decreases the pH of the aqueous feed below 12.5, the waste stream would no longer be considered hazardous under the EPA corrosivity classification D002 of the Resource Conservation and Recovery Act and disposal costs would be expected to decrease accordingly.
The present invention (i.e., the liquid-liquid extraction and recovery process) represents a potentially improved strategy, with respect to the prior art, for recovery of hydroxide from a caustic aqueous salt mixture. By reference to the hydrometallurgical industry, solvent extraction is generally recognized as providing economical, selective, high-throughput methodology and is widely practiced for a variety of separations. By analogy, the process presented here is expected to offer these same potential advantages. Experience with employing solvent extraction in the nuclear industry has also been extensive and productive. The use of solvent extraction for nuclear applications has lain overwhelmingly on the acid side, while applications for alkaline-side separations have only recently been considered. Nevertheless, in the case of technetium separation from alkaline tank waste, recent work has set the precedent that efficient, cost-effective processes are not only feasible and demonstrable by solvent extraction, but also highly selective (U.S. Pat. No. 5,443,731).
An ideal liquid-liquid extraction process would require no prior adjustment of the waste stream, consume no chemicals, and add no additional chemical substances to either the exiting depleted waste or alkali metal product streams. Use of water for stripping is highly compatible with such a process, necessitating extractants that release alkali metal hydroxide upon contact with water. With the possible exception of simple evaporation, no further separation steps would be needed to allow subsequent use of the hydroxide-containing product. Hence, extractant systems that are particularly attractive must afford appreciable loading directly from the aqueous feed, effective stripping with water, and adequate selectivity for hydroxide. (The meaning of "adequate selectivity" naturally depends on the anticipated use of the product.) It is important to point out that practical extraction systems must ultimately possess other characteristics such as good phase disengagement, facile kinetics, negligible partitioning of the extractant to the aqueous phase, resistance to the formation of third phases, and stability toward alkaline conditions.
A liquid-liquid process useful for the selective recovery of hydroxide from an aqueous mixture employing organic solvents containing substituted phenols in combination with one or more extractive additives has been described and constitutes the prior art. In U.S. Pat. Nos. 3,598,547 and 3,598,548, Grinstead teaches the use of phenols for the selective liquid-liquid extraction of sodium and potassium from aqueous solutions having a pH of approximately 14. The solvent in this process is comprised of a substituted phenol and an extractive additive containing one or more polar groups (e.g., hydroxyl, oxy, amino, or cyano) dissolved in a hydrocarbon diluent. When this solvent is contacted with an aqueous solution of sodium or potassium values having a pH greater than ca. 12, the sodium or potassium values are extracted into the solvent phase. Following extraction, the loaded organic phase is stripped with water to yield an aqueous solution containing predominantly sodium or potassium hydroxide. However, widespread use of phenols in this capacity is somewhat limited due to the fact that they are slowly decomposed under strongly alkaline conditions. Also, the properties of these compounds result in recovery of sodium and potassium hydroxide in a single contact with pure water that is less than quantitative. In the present invention, organic solvents containing fluorinated alcohol extractants in place of phenols are employed in an analogous liquid-liquid process. As described in detail below, these fluorinated extractants offer some distinct advantages over phenols, resulting in an improved process for the recovery of hydroxide from aqueous mixtures.